Conventionally, the illumination of an electrical device is done with backlighting a key or a keyboard. One type of backlit keyboard is fabricated by moulding a keypad device, including the keys and pad thereof, of light-transmitting rubber. An opaque character is formed on the top of each key by an opaque ink or the like. Light can also be transmitted through light-conducting channels formed in a keypad. Another type of backlit keypad device passes light only through character forming openings at the top of the keys, this second type being called as “reverse graphics illumination”.
Such solutions for illumination of an electrical device are presented for example in U.S. Pat. No. 5,708,428, GB 2 344 564 A, U.S. Pat. No. 5,746,493, U.S. Pat. No. 4,124,879 and U.S. Pat. No. 5,510,782. In U.S. Pat. No. 5,708,428 the light-emitting diodes are embedded, preferably by insert moulding, directly into translucent elastomeric material forming a keypad or surrounding structure adjacent the light guide of a liquid crystal display panel. Light is diffused into the translucent keypad and dispersed in a manner that provides backlighting for each of the keys of the keypad.
In GB 2 344 564 A is presented a luminous body for a mobile phone assembly, comprising at least one plane lightguide element adapted to distribute light within the mobile unit assembly, at least one distance element to mount the plane light guide element to a circuit board with a predefined distance therebetween, wherein the distance element is formed with a cavity to accommodate a light source. With this arrangement another type of backlighting is achieved.
In U.S. Pat. No. 5,746,493 is presented a more effective lightguide to distribute a light from light source to a display and keyboard. A plurality of light coupling recesses are provided on the peripheral edge to transmit light into the light guide. The light coupling recesses are preferably accurately shaped, and more preferably semicircular to guide light from the light sources uniformly into the lightguide avoiding bright and dark regions. With this arrangement is achieved more homogenous backlighting to a device.
In U.S. Pat. No. 4,124,879 light is distributed around the keypad from a light source on the side of the keypad through light-conducting channels situated between the rows of keys. The channels are provided with matt and reflecting painted surfaces for directing the light. This arrangement is also used to achieve more homogenous backlighting to a device.
In U.S. Pat. No. 5,510,782 the keypad device includes one or more separate key portions or keys that each includes a shell of elastomeric material having top and side walls and forming a cavity therewithin. Each key also includes an insert of light transmitting material lying in the cavity, with the top of the insert supporting the top wall of the shell. Openings are formed in the top wall of the shell, preferably with laser beam, so light passing upwardly through the insert can pass through the openings to form an illuminated character. This backlighting solution is called “reverse graphics illumination.”
All mentioned prior art solutions are illuminating a key or a keypad from back side of the object to be illuminated so that the light passes at least partly through the illuminated object. In other words the illumination is arranged from under the illuminated object and called backlighting.
These prior art solutions have one or more of following disadvantages in being used with opaque materials such as metals: associated problems with laser drilled key caps, expenses of the laser drilling, difficulty in seeing characters in half light due to diminishing contrast, impossible use of non transparent keys, not uniform illumination of keys or keyboards, etc.